1. Field of the Invention
The invention concerns a system for the display of images in half tones on a monochrome or color matrix screen not designed for this purpose.
The invention applies to plasma screens, electroluminescent screens, fluorescent screens, light-emitting diode screens, liquid crystal screens, etc. In particular, it is well suited to matrix screens and can be used to obtain half tones without any deterioration in the rated electrooptical performance characteristics (appearance, contrast, color, angle of view, flicker etc.) of these screens.
In particular, the method applied makes it possible to obtain an extensive color range on a screen initially designed for eight basic colors.
The term half tones (or so-called "grey scales") refers to any state of luminosity or intermediate contrast between two extremes of the response of an electrooptical effect.
2. Description of the Prior Art
Various methods are used to obtain half tones. In a first method, a spatial distribution or framing is obtained by modulating the surface of the excited zone, for example by sub-dividing the pixel into one or more zones of different areas or, more simply, by lighting up only one pixel out of two. The drawback of this method is that it increases the technological complexity of the screen or reduces the definition of the image.
In another method, the intensity of the electrooptical effect is modified by modulating the amplitude of the voltage applied to the electrooptical material or the period for which this voltage is applied. This method, which is the one commonly used, requires more complicated and more efficient screen control circuits than those used for screens without half tones.
In a third method, a temporal modulation is produced by exciting, for example, every other pixel to obtain, by temporal means, an intermediate luminance level. This causes an increase in flicker.
The aim of the invention is the application of a method of temporal modulation which does not produce any flicker and which, as shall be seen, uses the screen control circuits which are not normally designed for this purpose. The modulation is done at the level of an image memory associated with the screen and no longer at the level of the screen-addressing peripheral circuits.